What affects manufacturing costs?

I've been noticing that as I progress through my game on Classic Ironman, the cost for me to manufacture satellites keeps going down. It was §100 when I started the game, at another point it was §78, and now it's down to §70.
What affects the cost of manufactured goods? Number of engineers? Workshops? Something else?

Cost discount falls off quickly with number of engineers, and is capped at 50%. The formula is
Project cost = Base cost * (0.5 + 0.5 * (Minimum engineers / Available engineers)) That is,
 with double the required engineers you get 25% discount,
 with four times the required engineers you get 37.5% discount,
 with infinity engineers you get 50% discount.
Also, there is a 7% rebate for adjacent workshops, but AFAIK, that is returned after the project completes.